Most older tug boats and other small marine vessels such as yachts utilize a single inboard engine with a mechanical drive train to operate a single screw. More modern tug boats now more commonly have a single in-board engine with a mechanical drive train to operate twin screws.
A so-called hybrid tug has a single engine but utilizes an electrical transmission to drive twin screws. An electrical transmission takes rectified (“DC”) electrical output from an alternator and drives a DC or AC motor which turns a screw. This is directly analogous to a well-known diesel-electric locomotive power train. In one configuration, two different size engines can be used with an electrical transmission. The larger can be used, for example, when more power or higher speeds are required and the smaller engine can be used, for example, when less power and longer range are required.
The Terra Nova Marine Company has developed a commercially viable diesel-electric propulsion system for medium-sized fishing vessels utilizing multiple engines. Electric motors replace the main diesel propulsion engine so that the power production can be split to several smaller diesel generators. Electric motors are highly efficient over a range of operational speed and power output, while a diesel engine has a clear peak in efficiency. They estimate that the diesel-electric configuration will reduce emissions compared to a similar vessel with a conventional propulsion system.
A jet drive is commonly used on some small marine craft. The jet drive is comprised of an engine-driven water pump that sucks in water through the bottom of the boat and shoots it out through a nozzle at the stern. Steering the boat is done by changing the direction of the discharge nozzle. A few of the advantages in using a jet drive system are (1) a complete absence of all underwater appendages—no rudder, no propeller, no strut, and no shaft; (2) jet boats are safer around people in the water because of the lack of all underwater appendages, especially the propeller; and (3) because the system is so simple, there is less likelihood of failure. Among the disadvantages of jet drives are that they are still not as efficient as other systems and debris in the water (weeds, trash, etc.) can be sucked into the pump and cause the propulsion system to shut down.
Known to the applicant is WO1997/040999 which discloses a method and apparatus for propelling a sail-powered marine vessel using different power sources and power supplies.
There is an unmet need for a small marine craft drive architecture that can utilize multiple engines, energy storage and use jet drive technology for added maneuverability and regenerative braking.